1. Field of the Invention
The present invention relates to a projector in which an optical compensation sheet is disposed between a liquid crystal panel and a polarizing plate.
2. Description of the Related Art
FIGS. 11A and 11B are exploded perspective views of a conventional liquid crystal panel 7 and polarizing plates 73 and 74 sandwiching the liquid crystal panel 7. FIG. 11A shows a state where no electric field is applied to the liquid crystal panel 7, and FIG. 11B shows a state where an electric field is applied to the liquid crystal panel 7. As it is known, the polarizing plates 73 and 74 permit passage of one of two pieces of polarized light orthogonal to each other, and have vibration planes of polarized light which is permitted to pass. The vibration planes are deviated from each other by 90 degrees.
The liquid crystal panel 7 is constructed by sealing liquid crystal molecules 90 in transparent display substrates 70 and 71. The display substrates 70 and 71 are subjected to a rubbing process, and the stick-shaped liquid crystal molecules 90 are confined in a twisted state between the display substrates 70 and 71.
When an electric field is not applied between the display substrates 70 and 71, as shown in FIG. 11A, polarized light passed through the polarizing plate 73 on the incidence side passes the polarizing plate 74 on the outgoing side while being turned by 90 degrees by the twist of the liquid crystal molecules 90. Therefore, a light image, that is, a white image is displayed on the liquid crystal panel 7.
When an electric field is applied between the display substrates 70 and 71, as shown in FIG. 11B, the liquid crystal molecules 90 are arranged vertically, and polarized light passed through the polarizing plate 73 on the incidence side passes in gaps among the liquid crystal molecules 90. Since the polarized light is blocked by the polarizing plate 74 on the outgoing side, a black image is displayed on the liquid crystal panel 7.
It is, however, known that, actually, the tilt angle of the liquid crystal molecules 90 changes continuously in the thickness direction of the liquid crystal panel 7 in the liquid crystal panel 7 in a state where an electric field is applied between the display substrates 70 and 71 as shown in FIG. 12. It is known that, in a state where an electric field is applied between the display substrates 70 and 71, light leaks due to birefringence of the liquid crystal molecules 90 positioned near the substrates 70 and 71, contrast deteriorates.
In recent years, higher packing density of an image is in demand. An apparatus for capturing a high-density image has to improve contrast between black and white in a projected image and obtain a clear image. As described above, when polarized light which is inherently to be blocked passes through the liquid crystal panel 7 due to birefringence of the liquid crystal molecules 90 as shown by an alternate long and short dash line as shown in FIG. 11B, a black image is not displayed at all. The birefringence denotes here that travel speed of light varies according to the direction of the vibration plane of the light. The direction of fast speed will be called a fast axis, and the direction of slow speed will be called a slow axis.
In view of this point, as shown in FIG. 12, the technique of providing optical compensation sheets 8 and 8a in which liquid crystal molecules 91 are arranged in the thickness direction between the liquid crystal panel 7 and the polarizing plates 73 and 74 on the incident/outgoing sides is proposed (refer to U.S. Patent Publication No. 2002/18162). The optical compensation sheets 8 and 8a are transparent sheets in which the liquid crystal molecules 91 each having an almost disc shape are arranged.
The tilt angle of the liquid crystal molecules 91 changes continuously in the thickness direction of the sheet, and the outermost liquid crystal molecules 91 are arranged almost horizontally. With the arrangement, the birefringence of the liquid crystal molecules 90 in the liquid crystal panel 7 is compensated, and light leaked from the liquid crystal panel 7 does not pass through the polarizing plate 74. Therefore, a black image is displayed perfectly on the liquid crystal panel 7 and the contrast can be enhanced.
The alignment direction of the liquid crystal molecules 91 in the optical compensation sheets 8 and 8a has to be parallel with that of the display substrates 70 and 71.
However, there is a case that the alignment direction of the liquid crystal molecules 91 in the optical compensation sheets 8 and 8a is not parallel with that of the display substrates 70 and 71 due to an error of mounting of the optical compensation sheets 8 and 8a to the polarizing sheets 73 and 74.
As shown in FIG. 13, the optical compensation sheet 8 is cut in a necessary size from a film sheet 85. There is a case that the cut line is erroneously deviated from a normal position as shown by broken lines.
Such an optical compensation sheet 8 also transmits light to be inherently blocked since the alignment direction of the liquid crystal molecules 91 is deviated from the inherent position. As a result, a problem occurs that partial light leak occurs in a portion to be inherently displayed in black. Since the optical compensation sheet 8 is fixed, in the case where contrast deteriorates or display unevenness occurs, the optical compensation sheet 8 is replaced with another optical compensation sheet 8 having a different optical axis. Therefore, it is necessary to prepare optical compensation sheets having various optical axis, and it causes a problem such as occurrence of unnecessary stocks.
The present invention is directed to solve the problems.